On the Accuracy of Quantitative Measurements in Image Intensifier Systems

The image intensifier (II) tube is an electrooptical device used to detect, intensify, and shutter optical images. It is a vacuum tube that contains four basic elements: input phosphor screen and photocathode, electrostatic focusing lens, accelerating anode, and output phosphor. In diagnostic radiology, image intensifiers are applied in fluoroscopy and angiography where the viewing of images in real time is desired. This means that the X-radiation pattern emerging from the patient has to be transformed immediately into an image directly viewable by a radiologist.

In angiographic quantitative analysis, measurement of blood vessel diameters plays an important role and often also serves as a basis from which other values or indexes can be derived. This requires a technique that minimizes the distortion due to the structure of the II tube and permits a corrected image to be archived.

Image-intensifier tubes, built with electron focusing lenses, may have five aberrations: distortion of image caused by the input phosphor screen, astigmatism, curvature of image field, spherical aberration, or coma. Aberration caused by the curvature of an input phosphor surface changes the shape of images more than any other type of distortion. The discrepancy in size may reach even 20% in the periphery. Thus, the accuracy of measurement of abnormalities depends on its location within the image. Pathology viewed in the image periphery appears wider than that viewed in the central part of the image.

The aberration caused by the spherical surface of the input phosphor screen introduces a nonlinear relationship between points in the object plane and the corresponding points in the image. The error, generally referred to as the pincushion distortion, depends on the distance from the center of the input phosphor screen (the point in which the plane is perpendicular to the central X-ray), the radius of input phosphor screen curvature, and the view angle between the central X-ray and the axis perpendicular to the object plane.

There is one more type of distortion (discussed in Section 5.2) that originates from the same source. It results in brightness nonuniformity. Image periphery is denser than its center. The illumination decrease is caused by the spread of the X-ray beam over a larger surface. Since the light intensity is related to the size of the exposed area, the pixel value depends on the distance from the image center.

The increase of the surface area deteriorates the sharpness in the periphery. Edges viewed off the image center become blurred. After shrinking the image (performed by the correction function), enhancement is obtained.

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